The invention relates to a process and apparatus for pulsing, i.e., oscillating, a high velocity liquid jet at particular frequencies so as to enhance the erosive intensity of the jet when the jet is impacted against a surface to be eroded. Eroding conditions include cleaning, cutting, drilling or otherwise acting on the surface. The method may be particularly applied to enhance cavitation in a cavitating liquid jet such as described in U.S. Pat. Nos. 3,528,704, 3,713,699 and 3,807,632 and U.S. Pat. No. 4,262,757. It may also be used to modulate the velocity (at particularly preferred frequencies) of a simple high velocity liquid jet exiting in a gas in such a way as to cause the jet to become a series of slugs or drops which upon impact produce water hammer blows to the surface to be eroded.
In U.S. Pat. Nos. 3,713,699 and 3,807,632, cavitation, that is, the formation of vapor cavities or bubbles in a high velocity liquid jet in the shear zone between a high velicity jet and a relatively low velocity fluid, which surrounds the jet when the jet is either naturally or artificially submerged, is described as an important source of the vapor cavities in the jet. Furthermore, the patents disclose the concept of pulsing the jet.
Experiments have been reported using air jets discharging into a gaseous atmosphere. See, S. C. Crow and F. H. Champagne, "Orderly Structure in Jet Turbulence", Journal of Fluid Mechanics, Vol. 48, Part 3, August 1971. These experiments related to understanding the production of jet aircraft noise, and revealed that when the jet exit velocity, V, is oscillated about its mean value with an amplitude equal to only a few percent of the mean value, the structure of the jet altered dramatically when the frequency of oscillation (f) was in the range of 0.2 to 1.2 times the ratio of the jet velocity, V, to the jet diameter, D. That is, the jet structure change occurred for a range of Strouhal numbers, S, defined as (fD/V), between 0.2 and 1.2. The most dramatic change in the jet structure occurred for S=0.3 and 0.6. The shear zone surrounding the air jet apparently changes from a zone of largely uncorrelated fine scale eddies to a series of discrete vortices convecting down the periphery of the jet at a speed approximately equal to 0.7 of the jet exit speed. These vortices therefore have a spacing of approximately the jet diameter and appear to an observer stationary with respect to the nozzle exit as waves having a wavelength of the same order as the vortex spacing. This well-defined structure of the air jet is observed to break up after several jet diameters into a turbulent flow.
U.S. Pat. No. 3,398,758 discloses an air jet driven pure fluid oscillator as a means of providing a pulsating jet as a carrier wave for a communication device.
In "Experimental Study of a Jet Driven Helmholtz Oscillator," ASME Journal of Fluids Engineering, Vol. 101, September 1979, and U.S. Pat. No. 4,041,984, T. Morel presents extensive information on air jet driven Helmholtz oscillators and indicates that he was not able to achieve satisfactory operation for jet speed to sound speed ratios (Mach number) greater than 0.1.
U.S. Pat. No. 4,071,097 describes an underwater supersonic drilling device for establishing ultrasonic waves tuned to the natural frequency of rock strata. This device differs from the oscillators described by Mr. Morel or in U.S. Pat. No. 3,398,758, in that the resonance chamber is fed by an orifice which has a disturbing element placed in the orifice so as to partially obstruct the orifice.
U.S. Pat. No. 3,983,940 describes a method and apparatus for producing a fast succession of identical and well-defined liquid drops which are impacted against a solid boundary in order to erode it. The ultrasonic excitation of the liquid jet is accomplished with a magnetostrictive ultrasonic generator having a wavelength approximately equal to the jet diameter.
U.S. Pat. No. 3,405,770 discloses complex devices for oscillating the ambient pressure at the bottom of deep holes drilled for oil and/or gas production. These devices oscillate the ambient pressure at a low frequency (i.e., less than 100 Hz). The purpose of such oscillations is to relieve the overbalance in pressure at the hole bottom, so that chips may be removed; thus increasing the drilling rate.